Firefighters sift through the remains of a burned out house in Washington State after an explosion that killed a man and his two sons.

The house exploded moments after a child protective services worker brought the boys, aged 5 and 7, to their father Josh Powell.

Investigators say they believe Powell killed himself and his children on purpose.

(SOUNDBITE) (English) DETECTIVE ED TROYER, PIERCE COUNTY SHERIFF'S DEPARTMENT:

"We believe he intentionally started the fire because it was a scheduled visitation. The CPS worker came here, dropped the kids off. Locked the CPS worker out, ran in with the kids. The fire started immediately. It went very fast, very hot, burned very quick. So we believe there were accelerants used - the fire department does. It was set intentionally. We haven't confirmed the bodies but at this point we don't believe it to be anybody else rather than Josh Powell and his kids. And we believe he intentionally did it."

Custody of Powell's children was awarded to his wife's parents after his father was arrested on charges of possessing child pornography.

Powell's wife Susan went missing in Utah in 2009 - her husband was a "person of interest" in her disappearance.

Powell's attorney said his client sent him an email saying "I'm sorry, goodbye", moments before the explosion.

Simon Hanna, Reuters.